The present invention relates to rotary fluid pressure devices of the type in which a gerotor gear set serves as the fluid displacement mechanism, and more particularly, to such devices which are provided with multiple speed ratio capability.
Although the teachings of the present invention can be applied to devices having fluid displacement mechanisms other than gerotors, such as cam lobe type devices, the invention is especially adapted to gerotor devices and will be described in connection therewith.
Devices utilizing gerotor gear sets can be used in a variety of applications, one of the most common being to use the device as a low-speed, high-torque (LSHT) motor. One common application for low-speed, high-torque gerotor motors is vehicle propulsion, wherein the vehicle includes an engine driven pump which provides pressurized fluid to a pair of gerotor motors, with each motor being associated with one of the drive wheels. Those skilled in the art will be aware that many gerotor motors utilize a roller gerotor, especially on larger, higher torque motors of the type used in propel applications, and subsequent references hereinafter to xe2x80x9cgerotorsxe2x80x9d will be understood to mean and include both conventional gerotors, as well as roller gerotors.
In recent years, there has been a desire on the part of the vehicle manufacturers to be able to provide both the low-speed, high-torque (LSHT) mode of operation, such as when the vehicle is at the work site, and also a high-speed, low-torque (HSLT) mode of operation, for when the vehicle is traveling (xe2x80x9croadingxe2x80x9d) between work sites. One possible solution has been to provide a gerotor motor having a two-speed capability.
Two-speed gerotor motors are known from U.S. Pat. No. 4,480,971, assigned to the assignee of the present invention and incorporated herein by reference. The device of the cited patent has been in widespread commercial use and has performed in a generally satisfactory manner. As is well known to those skilled in the art, a gerotor motor may be operated as a two-speed-ratio device by providing valving which can effectively xe2x80x9crecirculatexe2x80x9d fluid between expanding and contracting fluid volume chambers of the gerotor gear set. In other words, if the inlet port communicates with all of the expanding chambers, and all of the contracting chambers communicate with the outlet port, the motor operates in the normal LSHT mode. If some of the fluid from the contracting chambers is recirculated back to some of the expanding chambers, the result will be operation in the HSLT mode, which is the same result as if the displacement of the gerotor were decreased, but with the same flow rate through the gerotor.
Although the two-speed gerotor motors which are in use commercially have been generally satisfactory, there have been certain inherent limitations present in these motors. The primary limitation in the known two-speed gerotor motors relates to the speed ratios available. For example, if the displacement mechanism of the motor is an 8/9 gerotor, in which the star has eight external teeth and the ring has nine internal teeth, and if the motor is designed to have every other volume chamber recirculate, then the available speed ratios are 1.0:1 (LSHT) and 2.0:1 (HSLT).
Therefore, in the general case, the ratio in the HSLT mode is the total number of volume chambers divided by the number of volume chambers which are xe2x80x9cactivexe2x80x9d, i.e., which do not recirculate. In order to provide two different motor models, each having a different HSLT ratio, it has been necessary, when utilizing the prior art, to change the number of volume chambers which recirculate, from one model to the next, thus necessitating a major change in the design of at least a portion of the motor.
Accordingly, it is a primary object of the present invention to provide an improved multiple speed ratio arrangement, especially suited for use with a gerotor motor, which results in greater flexibility in the choice of HSLT speed ratios.
It is a more specific object of the present invention to provide such an improved multiple speed ratio arrangement which accomplishes the above-stated object, without the need for any substantial redesign of the motor in order to be able to provide different models having different HSLT speed ratios.
Another functional limitation which has been inherent in the prior art two-speed gerotor motors is simply the fact that these motors have effectively been limited to two different speed ratios, i.e., the 1.0:1 low speed ratio with no volume chambers recirculating and the HSLT speed ratio determined by the number of volume chambers which are recirculating, as described above. Increasingly, there are vehicle applications in which it is recognized as being desirable to have more than just two speed ratios available.
Accordingly, it is another object of the present invention to provide an improved multiple speed ratio arrangement which accomplishes the above-stated objects and which further has the capability of providing at least a third speed ratio.
Finally, as is well know to those skilled in the art, it is desirable on many vehicles of the type which are propelled by hydraulic motors that the vehicle be capable of being towed. In order for the vehicle to be towed, however, the motors which propel the vehicle must be capable of operating in a xe2x80x9cfree wheelxe2x80x9d mode, or else towing the vehicle and causing the motor to operate as a pump will cause the fluid to overheat and may result in damage to the motor. As is also well known to those skilled in the art, when the fluid overheats, it begins to lose its lubrication capability, which is a primary reason for damage to occur to various parts of the motor.
One way to provide the free wheel capability in the motor, so that the vehicle can be towed, is to provide the propel circuit valving with a bypass feature. Therefore, with the propel circuit valving in a bypass condition, fluid can flow to and from the motor, through the valving, but with relatively little restriction to fluid flow. Unfortunately, adding such bypass capability to conventional propel circuit valving adds substantially to the overall cost and complexity of the valving, and of the overall propel circuit.
Accordingly, it is still another object of the present invention to provide an improved gerotor motor having a multiple speed arrangement which accomplishes the above-stated objects while at the same time providing the motor with free wheel capability, but without the added cost and complexity in the propel circuit necessitated by the prior art solution.
The above and other objects of the invention are accomplished by the provision of a rotary fluid pressure device comprising a housing defining a fluid inlet port and a fluid outlet port. A fluid pressure operated displacement means is associated with the housing and includes a first internally toothed ring member and a first externally toothed star member eccentrically disposed within the first ring member for relative orbital and rotational movement therein to define a plurality N+1 of expanding and contracting first fluid volume chambers in response to the orbital and rotational movements. A commutating valve means cooperates with the housing to provide fluid communication between the inlet port and the first expanding volume chambers and between the first contracting volume chambers and the outlet port. A shaft means is included for transmitting the rotational movement of the first star member.
The improved device is characterized by the fluid pressure operated displacement means including a second internally toothed ring member and a second externally toothed star member eccentrically disposed within the second ring member for orbital and rotational movement therein, to define a plurality N+1 of expanding and contracting second volume chambers in response to the orbital and rotational movements. The device includes connection means for connecting the second star member to the first star member for common orbital and rotational movement therewith. A selector valve means is operably associated with the first and second ring members and is operable in a first low speed position to permit fluid communication to each of the first volume chambers, and its corresponding second volume chamber, and a second high speed position in which the commutating valve means permits communication of pressurized fluid to only one of the first and second volume chambers, and permits fluid communication between each of the other of the first and second volume chambers and a fluid recirculation chamber.